Circuit arrangements for identifying network zero crossings of a mains voltage of an alternating current network are used wherever a signal synchronous to the AC voltage is needed.
One area of application is the in-phase control of power sections of a converter or of an inverter as a function of zero crossings of the mains voltage.
In such cases the converter or inverter is typically used to provide electrical energy in the form of an alternating current which is suitable for feeding into the alternating current network. One area of application for this is the connection of alternate power sources to a mostly public alternating current power network. Such power sources can be solar panels, fuel cells, wind power generators or other power sources such as rechargeable batteries.
According to the prior art there are numerous known converter circuits with different possible applications. Different basic types of electronic converter such as boost converters, buck converters or boost-buck converters are used to generate a half-wave current, combined with full or half bridges for inverting each second half wave resulting in a single-phase or multi-phase current.
In photovoltaic systems in particular different demands of the network operator and the authorities must be fulfilled, for example the supply of a sine-wave current. The sine-wave form is in such cases generally derived from the mains voltage.
An important criterion for a network-synchronous supply is the precise detection of the network zero crossing points for in-phase control of the power sections. The zero crossings in the mains voltage define for the inverter control the times at which the switchover is to be made from a positive to a negative half wave and vice versa.
The in-phase control of the power parts of an inverter is also important for the safety of the device however. An incorrect switchover from a positive to a negative half wave or vice versa leads without appropriate safeguarding measures to a short circuit in the power section of the inverter, which can result in the power components being damaged or destroyed.
A known circuit arrangement for identifying zero crossings consists of a voltage divider and a comparator or operational amplifier (cf. FIGS. 1 and 2) In this arrangement a square-wave signal is present at the output of the comparator or operational amplifier, with for example its low value showing a negative half wave and its high value a positive half wave of the mains voltage. The signal value thus changes with each zero crossing of the mains voltage. The resistors of the voltage divider are at high resistance in this case in order to keep the power dissipation low.